Add dotenv for environment variables

We talked briefly about this at the last Codebar session. I added it in when I started looking into the S3 issues. I hope you don't mind me submitting this!

There's two benefits to using dotenv:

1. Environment variables are kept out of your `.zshrc` file. This keeps things cleaner and also avoids the security risk of anything running in a shell having access to your AWS credentials.

2. It self documents the environment variables that your app requires. I searched the code to try and find out what the app needs. I think I got them all!

To use it copy `.env.example` to `.env` and fill in the correct values.